Deposition of Polypyrrole into Porous Silicon
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lime / s Figure 2. Chronopotentiograms for the formation of polypyrrole from a 0.05 M monomer solution into a 3[tm thick PS sample under different anodic current densities: (a) 16 mA/cm2; (b) 8 mA/cm2; (c) 4 mA/cm2; (d) 2 mA/cm2. Figure 2 shows the influence of the current density of the anodic pulse on the E-t curves obtained from a 0.05 M pyrrole solution. The time interval CD, which is the transition between the two constant potential regions, takes place at longer times as i decreases. At the same time, the obtained E values are smaller. The amount of polymer formed inside the pores must be directly related to the electric charge transferred in the time interval BC, umc. Measurements made at different anodic current densities show that qBc decreases when i increases, once a limiting value for i is surpassed. This fact seems to indicate that pores are not being completely filled by polymer, which would lead to deficient electrical contacts. For this reason, subsequent Raman experiments were carried out with samples where polypyrrole was electroformed at a relatively low i (4 mA/cm 2 ). Figure 3 shows a Raman spectrum of a PS sample containing polypyrrole. The spectrum of polypyrrole deposited directly on monocrystalline silicon has been superimposed. Both spectra present the same bands for the polymer. Figure 3 also shows results for pure monocrystalline silicon, where only a Si peak at 520 cm-' is obtained. It should be noticed that the latter peak is broader in the tails when it is obtained from PS. This effect is due to a relaxation of the selection rules originated by the small size of PS nanocrystals. In Figure 4, the Raman intensity of the polypyrrole peak at 901 cm-1 has been plotted against the depth of the porous layer. In this way, data about the relative quantity of polymer in each section of the porous layer can be gained. It is particularly interesting to ascertain whether there is polymer at the bottom of the pores (and therefore, no void is left at the inner part of the structure), so as to study its growth along time interval BC.
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Figure 3. Raman Spectra of (a) polypyrrole into PS, (b) polypyrrole on monocrystalline Si, and (c) bare monocrystalline Si.
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E Depth / ýam Figure 4. Raman results for polypyrrole formed into PS from a 0. 1 M pyrrole solution; (a) at 4 mA/cm2 stopping in the middle of the BC time interval; (b) at 4 mA/cm2 stopping at D; (c) at 16 mA/cm2 stopping after D. Depth resolution is ca. 1 tim.
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Plot a in Figure 4 corresponds to a 12 tim thich PS sample in which polypyrrole was formed by means of an anodic pulse at 4 mA/cm2 that was stopped just in the middle of time interval BC. The presence of polymer at the bottom of the pores can be noticed, although the greatest quantity is found in an intermediate region. Plot b correspond
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